The invention relates to four-quadrant current-regulated systems using pulse-width modulation, especially for example for the control and current regulation of D.C. motors up to 10 kW. With four-quadrant energization, the motor is connected in the diagonal of a four-switch bridge. To establish motor current in a first or the opposite second direction, one renders conductive one pair of diagonally opposite switches, or else the other pair of diagonally oposite switches. In such circuits, it is known to use either transistors or resettable thyristor valves for the four switches. Resettable thyristor valves are quite expensive, and comprise a plurality of thyristors, capacitors, diodes and chokes, and accordingly these tend only to be used when the power to be handled exceeds the capability of high-power transistors. However, even when less expensive high-power transistors can be used, the cost for these is still quite high.